Geoscience Reference
In-Depth Information
Main Atmospheric Absorption Bands
O
3
O
2
H
2
O
CO
2
H
2
O
O
3
CO
2
H
2
O
3500
Black Body Radiation at 6000˚K (1.6 kWm
-2
)
Incoming Solar
Extraterrestrial solar
Radiation
(1370Wm
-2
)
2000
Outgoing Terrestrial
1000
500
0
5
10
15
20
µ
m
Direct Beam (Normal Incidence) Solar Radiation
at the Earth's Surface (907Wm
-2
)
200
Black Body Radiation 300˚K
(468Wm
-2
)
100
50
Estimated Infra-red
Emission to Space
from the Earth's
Surface (70Wm
-2
)
10
µ
m
Figure 3.1
Spectral distribution of
solar and terrestrial radiation, plotted
logarithmically, together with the main
atmospheric absorption bands. The
cross-hatched areas in the infra-red
spectrum indicate the 'atmospheric
windows' where radiation escapes
to space. The black-body radiation at
6000 K is that proportion of the flux
which would be incident on the top of
the atmosphere. The inset shows the
same curves for incoming and outgoing
radiation with the wavelength plotted
arithmetically on an arbitrary vertical
scale.
Source
: Mostly after Sellers (1965).
12
µ
m
20
10
5
Black Body
Radiation
263˚K
(270Wm
-2
)
2
1
01
02
05
10
20
50
10
20
50
10
Wavelength (
µ
m)
Ultraviolet
Visible
Infrared
at the maximum rate possible for a given temperature.
The energy emitted at a particular wavelength by a
perfect radiator of given temperature is described by
a relationship due to Max Planck. The black-body
curves in Figure 3.1 illustrate this relationship. The area
under each curve gives the total energy emitted by
a black body (
F
); its value is found by integration of
Planck's equation, known as Stefan's Law:
the energy received is inversely proportional to the
square of the solar distance (150 million km). The
energy received at the top of the atmosphere on a surface
perpendicular to the solar beam for mean solar distance
is termed the
solar constant
(see Note 1). Satellite
measurements since 1980 indicate a value of about
1366 W m
-2
, with an absolute uncertainty of about
2 W m
-2
. Figure 3.1 shows the wavelength range of
solar (short-wave) radiation and the infra-red (long-
wave) radiation emitted by the earth and atmosphere.
For solar radiation, about 7 per cent is ultraviolet
(0.2-0.4 µm), 41 per cent visible light (0.4-0.7 µm)
and 52 per cent near-infra-red (>0.7 µm); (1 µm =
1 micrometre = 10
-6
m). The figure illustrates the
black-body radiation curves for 6000 K at the top of
the atmosphere (which slightly exceeds the observed
extraterrestrial radiation), for 300 K, and for 263 K. The
F
=
σ
T
4
where
10
-8
W m
-2
K
-4
(the Stefan-Boltzmann
constant), i.e. the energy emitted is proportional to the
fourth power of the absolute temperature of the body (
T
).
The total solar output to space, assuming a temper-
ature of 5760 K for the sun, is 3.84
10
26
W, but only
a tiny fraction of this is intercepted by the earth, because
σ
= 5.67
